1. Field of the Invention
The field of the invention is the drying of porous materials such as textiles, paper and paperboards, certain foods and the like. More particularly, the subject of this invention is a drying rate testing apparatus capable of relating laboratory results to actual mill operations.
2. Prior Art
Papermaking is a capital intensive process and there is an appreciable economic incentive to increase utilization of this capital through increased productivity. The production rates on many paper machines, particularly among those producing heavy grades of paper or board, are limited by the rates at which water can be removed from the wet paper webs by evaporation. In these situations, it is common to minimize the evaporative load on the dryer by adjusting process and furnish variables, within the constraints of product quality, to enhance water removal by mechanical processes such as drainage or pressing. However, the effects that the properties of a wet web, such as composition and structure, have on the evaporation rates within the dryer can be appreciable. These effects should not be overlooked, but have in the past been difficult to predict because of limitations of the analytical techniques.
Most of the paper and board produced commercially is dried by threading a continuous wet web around each of a series of rotating, steam heated cylinders. The cylinders are generally arranged so that the wet web spends approximately 80% of the drying time in contact with the heated cylinders and approximately 20% of the drying time between cylinders in open draws. Almost all of the heat required for vaporization is transferred into the sheet during contact with the hot cylinder and the overall drying rate is directly dependent upon the overall resistance to heat transfer during contact. Any change in this resistance will be manifested as a corresponding change in the overall drying rate.
The overall resistance of the transfer of heat from steam to paper during contact is the sum of the resistances due to the condensing layer on the inside surface of the cylinder, the cylinder walls, the contact between the cylinder and the web, the paper web itself and the boundary layer of air adjacent to the open surface of the web. The open-mesh dryer fabrics normally used with most dryers offer negligible resistance to the transfer of heat.
It can be shown, in all but the lightest grades of paper, that the transfer of heat is limited primarily by the contact resistance and the internal resistance of the paper or board. It has been estimated that the sum of these two resistances typically accounts for between 50 and 90% of the total resistance to heat transfer in paper and paperboard grades.
Drying of a wet web is a complex process involving the concurrent and interactive flow of heat and mass through a structure which is itself responding to local changes in moistures and moisture gradients. This complexity is manifested in the complex dependence of the overall thermal resistance of the web to factors such as web composition and structure, drying conditions and drying history.
Elaborate models have been proposed to qualitatively describe the phenomenon of heat and mass transport within the sheet during prolonged contact with the hot dryer surface. However, present knowledge is generally inadequate to predict the magnitude and often even the direction of changes in drying rate on a drum dryer due to changes in process or furnish variables.
It has been shown that during prolonged contact with a hot plate, evaporation occurs from regions of the web close to both the open and closed surfaces. In heavy grades of paper and paperboard, distinct concentration gradients form through the thickness of the web with most of the water located in regions remote from both surfaces. This distribution of water more or less resembles the symmetrical distribution of moisture which must exist within a web during conventional drum drying, when both surfaces are alternatively exposed to either a hot surface or a drying atmosphere. The mechanisms controlling heat and mass transfer are, therefore, expected to be the same in both systems.
Though the economic stake in improving drying production is high, a practical analysis of the heated surface drying of thin porous sheets has not yet come to fore. The relatively small changes in weight of the wet paper during drying have been dfficult to accurately measure because of the typically heavy hot metal surface on which they are dried. Discontinuous measurement of change of weight of these kinds of samples during drying has, in general, yielded results of only limited usefulness because of insensitivity of the measuring techniques or a failure to control important process variables. Attempts to obtain significant correlation between lab results and mill processes have largely failed because of the difficulty of measurement.
T. K. Sherwood in "The Drying of Solids-II," Ind. Eng. Chem. 21(10):976 (1929) describes drying slabs of 1.52 cm thickness of sulfite pulp using a stream of hot air while using a recording balance to continuously measure weight change versus time. Since no heavy hot plate was used, the total weight to be measured permitted accurate measurement. However, air drying of wood pulp involves the countercurrent flow of heat and mass within the porous structure. Drying characteristics under these conditions do not resemble those of hot surface drying which involves the cocurrent flow of heat and mass. Consequently, results from air drying experiments have not been useful for predicting drying rates on drum dryers.
Smith and Attwood in "Paperboard Drying Investigation by Means of an Experimental Drying Machine," TAPPI 36(11):481 (1953) describe a complex mechanical device that is an attempt to simulate all phases in a conventional drum dryer. The maximum simulation speed of 240 ft per minute is low in comparison with real paper machine speeds. The simulation also fails with respect to determining drying rates because the method for measuring moisture content is discontinuous and therefore imprecise.
Dreshfield in "Hot Surface Drying of Fiber Sheets," Chem. Eng. Prg. 53(4):174 (1957) describes a beta ray transmission method for measuring the moisture content of fibrous webs on a hot plate. Moisture measurements are discontinuous and only vaguely describe the drying rate. As an alternative to more conventional techniques, the beta ray method is complicated and expensive and, therefore, impractical.
Kirk and Jones in "Hot Surface Drying of Paper," Paper Technology 11(5):347 (1970) describe a technique for measuring drying rate of a hot surface dryer, in which the wet sheet was intermittently separated from the hot surface, weighed and returned to the hot surface. While producing drying rate curves, the inaccuracy of the determination is particularly evident in the "falling rate" portion of the curves.